Typical binary or quadrature phase modulators use a passive double balanced mixer device to perform the phase modulation. Double balanced mixers require a bipolar data signal with positive and negative voltage swings to switch the PIN diodes in the mixer. A bipolar signal is not commonly found in data generating circuits, but rather unipolar signals are normally used, where the voltage swings from a positive voltage to zero volts. Extra circuitry is needed to generate the bipolar signal for the double balanced mixers of the prior art phase modulators. Capacitor coupling a unipolar data signal to the double balanced mixers is a typical manner in which to generate the positive and negative data signal voltages to switch the PIN diodes in the double balanced mixers. However capacitive coupling impairs the low end of the frequency response of the modulator so that long strings of digital 1's or 0's do not properly modulate. In addition, double balanced mixers used for this application can have as much as 10 dB insertion loss.
It is desirable to have an antenna modulator, which has less than 1 dB insertion loss, which operates with a unipolar data signal and has a frequency response extending from dc up to the low MHz range. It is also desirable to have an antenna modulator in which the antenna is a probe fed patch antenna, which radiates the radio frequency signal while providing the phase shifting correlated to the input data stream.